Conventionally, various types of pressure sensors are proposed for detecting a pressure that is introduced to a sensing portion of the sensor from a pressure introduction port. For example, U.S. Pat. No. 5,595,939 discloses a pressure sensor for detecting a pressure of an object. FIG. 4 shows a cross-sectional view of the pressure sensor in the disclosure.
The pressure sensor shown in FIG. 4 is a diaphragm type pressure sensor that has a first case J1 and a second case (i.e., a housing J2) for respectively housing a connector and a pressure introduction portion. That is, the housing J2 is fastened by bending its flange onto the first case J1 having a metal diaphragm J3 interposed therebetween in a concave portion J4 for forming a pressure detection space J5 that houses a sensing element J6 and the like.
The pressure sensing space J5 is sealed by an O ring J7 that is interposed between the first case J1 and the housing J2, and the space J5 is filled with a pressure transfer medium J8 such as an oil or the like. The housing J2 has a pressure introduction hole J9 formed therein, and the hole J9 introduces the object into a subspace J10 that is formed in a horn shape (i.e., having a tapered cross section in an axial direction). The pressure of the introduced object is applied on an opposite side of the metal diaphragm J3 relative to the pressure detection space j5. The pressure of the object is conveyed and applied to the sensor element J6 through the pressure transfer medium J8. In this manner, the sensor element J6 outputs a detection signal to an external circuit through an aluminum wire J11 and a connector terminal J12 for detecting the pressure of the object.
As shown in FIG. 4, the pressure sensor introduces the object into a horn shape space (the subspace J10) for applying the pressure on a broad area of a pressure sensing portion (the metal diaphragm J3). This is because loss of the pressure is reduced when the pressure of the object is applied to a broad surface. In other words, this structure is effective when accuracy of the pressure sensor is in pursuit.
The pressure sensor having the above-described structure works appropriately when, for example, transmission of the pressure of the object introduced from the pressure introduction hole J9 is relatively slow. In this case, the pressure of the object is applied evenly on the metal diaphragm J3 as shown in FIG. 5A.
However, as shown in FIG. 5B, when the pressure of the object from the hole J9 transmits rapidly toward the diaphragm J3, or when increase of the pressure is steep and high in amount, the pressure is first applied to a center portion of the metal diaphragm J3, and then the pressure propagates radially toward a periphery of the diaphragm J3. In other words, there is a time lag between an arrival of the applied pressure at the center of the diaphragm J3 and at the periphery of the diaphragm J3.
Further, as shown in FIG. 5C, when the object is a mixture of gas and liquid or the like, the arrival of the applied pressure is observed at different times respectively at the center and at the periphery of the diaphragm J3 due to the gas and/or the liquid in the pressure introduction hole J9 and/or the subspace J10.
The applied pressure arriving to the pressure sensing portion at different times means that the pressure is unevenly applied on the pressure sensing portion (the surface of the metal diaphragm J3). In addition, when the pressure transmits in a liquid, pressure wave is not easily attenuated, thereby causing a reflected pressure wave that also causes unevenness of the pressure.
The unevenness of the applied pressure on the pressure sensing portion leads to a crack or a breakage of the metal diaphragm J3, or leads to a snapping of the aluminum wire J11 that connects the sensor element J6 and the connector terminal J12.